1. Field of the Invention
The present invention relates to a display device, a driving method of the display device, and an electronic apparatus and, more particularly, to a flat panel type display device having pixels including electro-optical elements disposed in a matrix shape, a driving method for the display device and an electronic apparatus using the display device.
2. Description of Related Art
In the field of display devices for displaying video and text data, a flat type display device in which pixels (pixel circuits) having electro-optical elements are disposed in a matrix shape has been developed recently and researched for marketability. This flat type display device includes an organic electro luminescence (EL) display device using an electro-optical element of a so-called current drive type where an emission luminance changes in response to a value of current flowing through the device, for example, an organic EL element utilizing a phenomenon where optical emission occurs when an electric field is applied to an organic thin film, as an electro-optical element of a pixel.
The organic EL display device consumes only a small power because the organic EL element can be driven at an application voltage of 10 V or lower. Further, since the organic EL element is an emissive element, the organic EL display device is characterized by higher visual recognition of an image, no backlight, faster response speed of an element and the like, as compared to a liquid crystal display device which displays video and text data by controlling a light intensity of a light source (backlight) at each liquid crystal cell of a pixel.
Similarly to a liquid crystal display device, an organic EL display device can adopt as its driving method a simple (passive) matrix method and an active matrix method. Although a display device of a simple matrix type has a simple structure, it is associated with a problem that a large and high precision display device is hard to realize. Therefore, vigorous development has been conducted in recent years for a display device of the active matrix type which controls current flowing through an electro-optical element by an active element provided in the same pixel circuit of the electro-optical element, such as an insulated gate type field effect transistor (generally a thin film transistor (TFT)).
It is generally known that the I-V (current-voltage) characteristics of an organic EL element deteriorate with the passage of time (deterioration in time).
In a pixel circuit which uses an n-channel TFT as a transistor for current driving an organic EL element (hereinafter called a “driver transistor”), the organic EL element is connected to the source side of the driver transistor. Therefore, as the I-V characteristics of the organic EL element deteriorate with the passage of time, a gate-source voltage Vgs of the driver transistor changes, and accordingly the emission luminance of the organic EL element changes.
This phenomenon will be described more specifically. A source potential of the driver transistor is determined by an operation point of the driver transistor and organic EL element. As the I-V characteristics of the organic EL element deteriorate, the operation point of the driver transistor and organic EL element varies. Therefore, even if the same voltage is applied to the gates of the driver transistors, the source potentials of the driver transistors become different. Since a source-driver voltage Vgs of the driver transistor changes, the value of current flowing through the driver transistor changes. Since the value of current flowing through the organic EL element changes, an emission luminance of the organic EL element changes.
In a pixel circuit using a polysilicon TFT, in addition to the deterioration in time in the I-V characteristics of an organic EL element, because of the change of a threshold voltage Vth and a mobility μ with the passage of time and manufacturing process variation (variation of transistor characteristics), a threshold voltage Vth and a mobility μ of a driver transistor change with time, and become different for each pixel. If threshold voltages Vth and mobilities μ are different among driver transistors, there arises a variation of values of currents flowing through the driver transistors. Therefore, even if the same voltage is applied to the gates of driver transistors, emission luminances of organic EL elements become different among the pixels, thereby degrading the uniformity of a display screen even though the same voltage is applied to the gate of the driver transistor.
A pixel circuit is provided with a compensation function for a change in the characteristics of an organic EL element and a correction function for a change in the threshold voltage Vth and mobility μ of a driver transistor, so as to maintain constant the emission luminance of the organic EL element, without being adversely affected by the deterioration in time in the I-V characteristics of the organic EL element and in the threshold voltage Vth and mobility μ of the driver transistor (e.g., refer to Patent Document 1: Japanese Patent Application Publication No. 2006-133542).